Friction stir joining has a problem that excess heating to members or pieces to be joined results in coarse joining quality. Patent Document 1 listed below therefore proposes a friction stir welding apparatus shown in FIG. 10. In a housing 110, a spindle 111 is supported rotatably through a bearing 112 and coupled to a drive motor not shown. A lower end of the spindle 111 is supported by a tool head bearing 113 fixed to the housing 110. A probe 115 located in a holder is connected with the spindle 111 and protrudes out through an opening 116 of a housing lower part 114. A non-rotating slide part 118 is supported in the opening 116 so that this slide part 118 slides on the surface of a joint without rotating during friction stir welding.